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VIDEO CONFERENCING

What is video conferencing?

Video Conferencing actually encompasses a range of technologies used in a wide range of situations, Often it is not just video and audio that is transmitted, but also data, allowing collaborative working through shared applications. Video conferencing may be: -

· One-to-one meetings, also known as point to point communications, usually involving full two way audio and video. · One-to-many, involving full audio and video broadcast from the main site, where other sites may be able to send audio. For example in a lecture situation, students can ask questions. · Many-to-many, known as multi-point communication, provides audio and video between more than two sites. With most multi-point systems only one site in a conference can be seen at time, with switching between sites either controlled manually or voice activated (i.e., the loudest site is on screen)

Physically, the most common scenarios of video conferencing are:

· Desktop video conferencing – usually a small camera is located on top of the PC or workstation monitor. The actual video is usually displayed in a small window, and shared applications, such as a shared white board are often used.

· Studio-based systems – a studio is specially equipped for video conferencing. This will normally include one or more cameras, microphones, one or more large monitors, and possibly other equipment such as an overhead camera for document viewing. Usually for more formal meetings. In practice a ‘studio ’may not be a dedicated room, but a standard seminar room with portable equipment that can be set up when required.

Bandwidth and Compression.

The bandwidth, or baud rate, is the amount of information, which can be transmitted every second. The higher the bandwidth, the better quality the signal that can be transmitted. For a video conference audio and video signals must be transmitted in real time, i.e., a lot of information has to be sent every second, requiring a very high bandwidth. A full screen image might be 640 * 480 pixels, over 7 million nits. For full motion video, the images are refreshed 25 times per second. This adds to over 184 million bits per second. It is not realistically possible to transmit this amount of information, and your PC certainly couldn’t receive at this rat. Therefore for digital video some form of compression is required.

Delivery

ISDN

Integrated Services Digital Network (ISDN) is offered by many telephone companies that provides fast, high-capacity digital transmission of voice, data, still images and full-motion video over the worldwide telephone network. In UK service providers such as British Telecom and wireless offer basic and primary rate ISDN services. Basic rate services provide 2 65kbit’s data channels or B channels and one control or D channel. This can provide reasonable quality video conferencing delivering about 10 frames per second. Primary rate access can carry 30 B channels and one D channel. This level of access will give good quality video and audio. ISDN is rapidly growing in popularity and is widely accepted in industry as the way to access multimedia over a network. Although it is still expensive when compared to a standard line, particularly for primary rate access, it may be suitable for inter-site conferencing.

IP

Video conferencing systems based on IP rather than ISDN offer several advantages, the main one being that many people already have a connection to an existing IP infrastructure. IP based system is the cheapest solution in many cases. The main disadvantage is bandwidth. Though often not a problem on an internal Local Area Network (LAN), IP videoconferencing across the Internet can be subjected to many delays, producing a poor frame rate and often unacceptable quality audio.

Satellite broadcast

Satellite transmission is usually used for one-to-many conferences, as described for cable. Although it is inexpensive, cost is not affected by distance, and therefore it may be of use where very large distances or many sites are involved.

Uses of Video conferencing:

· In general videoconferencing and screen sharing systems can be especially useful in networked organizations of all kinds and sizes, in large institutions scattered across remote locations, by freelance specialists who need to communicate with remote clients, in education for distance learning, in the medical profession and other situations where access to remote specialists is useful, and in scientific research where it is benefit to broadcast and discuss experiments and findings with a global community.

· It ties in very well with current organizational trends. One of these is the general drive for greater efficiency and flexibility. Videoconferencing enables collaborative work between remote groups without the need to waste time and money on travelling and scheduling meetings. Whereas it wouldn’t be worth travelling hundreds of miles for an hour meeting, with videoconferencing it is easy to have short discussions as required and deal with the problems as they arise

· Another trend is for people to work from home and videoconferencing is an ideal way for people to keep in touch, give progress reports and receive instructions. Much current work to improve the videoconferencing systems is aimed at the teleworker market by trying to make conferencing systems which will run both on ISDN or ordinary analog telephone lines and across Local Area Networks.

· Another organizational trend is to contract work out and hire freelancers for individual projects as required. Both for the freelancers and the core organization, videoconferencing is an ideal way to discuss a project without wasting time on travel. It means an organization has more freedom to hire the right person for the job regardless of their geographic location. It also means that freelance homeworkers can have a wilder client base.

· In education it has been used to link rural schools in Scotland, typically with only one or two children of each age group, allowing children to interact in classroom scenarios with more children of their own age. Other applications are for distance learning lectures and seminars.

Video and Audio encoding

Digital audio compression techniques

Uncompressed digital audio can require a large amount of bandwidth to transmit. There are many techniques used to compress digital audio. Some of the techniques commonly used in desktop videoconferencing systems are mu-law and A-law PCM, ADPCM.

Mu-law and A-law PCM

With PCM encoding methods, each sample is represented by a code word. Uniform PCM uses uniform quantizer step spacing. By performing a transformation, the quantizer step spacing can be changed to a logarithmic, allowing a larger range of values to be covered with the same number of bits. These are two commonly used transformations: mu-law and A-law. These transformations allow 8 bits per sample to represent the same range of values that would be achieved with 14 bits per sample uniform PCM. The mu-law and A-law PCM encoding methods are formally specified in the International Telecommunication Union.

ADPCM

PCM encoding methods encode each audio sample independently from adjacent samples. However, usually adjacent samples are similar to each other and the value of a sample can be predicted with some accuracy using the value of adjacent samples. The ADPCM (Adaptive Differential Pulse Code Modulation) encoding method computes the difference between each sample and it’s predicted value and encodes the difference.

Typically these above techniques can achieve real-time compression and decompression in software or inexpensive hardware. Some techniques apply to general audio signals and some are designed specifically for speech signals.

Digital Video compression techniques

Analog video is digitized so that a computer may manipulate it. Each frame of video becomes a two dimensional array of pixels. A complete color image is composed of three image frames, one for each color component. Uncompressed images and video are much too large to deal with and compression is needed for storage and transmission. Important metrics of compression is the compression ratio and bits per pixel.

Video compression is typically lossy, meaning some of the information is lost during the compression step. This is acceptable though, because encoding algorithms are deigned to discard information that is not perceptible to humans or information that is redundant. There are some basic techniques common to most video compression algorithms, including color space sampling and redundancy reduction.

Color space sampling is an effective technique used to reduce the amount of data that needs to be encoded. If an image is encoded in YUV space, the U and V components can be subsampled because the human eye is less sensitive to chrominance information.

Redundancy reduction is another technique used to decrease the amount of encoded information. Intraframe encoding achieves compression by reducing the spatial redundancy within a picture. Interframe encoding achieves compression by reducing the temporal redundancy between pictures. This technique works because neighboring frames in a sequence of images are usually similar.

One of the important techniques related to video conferencing is:

MJPEG

JPEG is an encoding standard for still images developed by the Joint Photographic Experts Group. Although designed for still images, with special hardware it is possible to encode and decode a series of JPEG images in real-time to achieve motion video. This use of JPEG encoding is typically referred to as Motion JPEG or MJPEG. However, no official MJPEG standard exists. There are four defined modes o

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